I think what you have is a chunk of vein or dyke, with some of the host rock adhering to one side and a piece engulfed within it. The host rock looks to be limestone, which has suffered rain weathering much more than the vein/dyke, hence it appearing smooth and drawn-back from the edges.

I can't tell from the piccies if the main part is vein or dyke. If it is vein, it could be a dark quartz (though there is also a separate small pale vein of what appears to be calcite). If it is a dyke, it would be basalt or similar.

To be sure, it would have to be looked-at in thin section under a microscope - to determine the minerals and textures, and thence to deduce the rock type(s). Ordinarily, we'd then find the source locality on a geological map, see what rocks are in the area, and see if it fits with one of them - though recognizing that loose rocks can be transported long distances by glaciers and rivers ... and people!

To me it looks like a part of one of the Earths plates, which crash together to make a mountain. This particular piece is definietly not a fossil, it is is a mineral (unless it is a stromatolite, but that is very unlikely.). I think the smoothness inside is where the rock started to melt from the heat when the mountain formed. You are definietly right in saying it is a natural rock ( not man made.). Presumably, but I may be wrong, when the two Earths plates crash together, it is going to create thermal energy, like rubbing two sticks together, which makes fire. Your piece looks rather burned, and with the smoothness inside, I am 70 percent sure it was once a part of the Earths plate, which formed the mountain, and the thermal energy from it melted the rock.

Here is it which is more easy to understand:

1. The Earths plates are moving closer to each other.

2. They clash together and the thermal energy melts the rock, hence the smoothness in the middle.

3. The Earths plates rise out if the ground due to the collision, creating a mountain.

Please note, I may be completely wrong with my theory- it is just what I believe to be the answer is: there is also a big possobility mike is right.